The ending with Leviathan and Extended Cut is not that bad. Did you only play it without the DLC? If not, what didn't you like about it?
ME3's galaxy is just as open as the other games really, you can generally do missions when you want and it retains the hub-based, sandbox gameplay for the most part. Squad mates still say very different things in the same situations, depending on how you handle the situation. ME3 continues the formula used in ME2, improving upon it mechanically. I always found it strange that people treat it like a vastly different game just because they didn't like the ending.
The ending actually bothered me from a different point of view.
To be clear, I really disliked the game (the only exception was geth consensus and geth/quarian conflict). In fact, I felt so out of place in ME3, that at some point I just youtubed the ending to figure out how it ends, because I really didnt want to play it anymore.
Nah, I played without the DLC (thhou I did watch the extended endings, they felt a bit lackluster. Hell, I even decided Its better to belive the Indoctrination Thoery). My problem wasent exactly "main story" beeing weak, but both in ME1 and ME2 you start small and you build up (weapons upgrades, more squad mates, etc). Then, when you reach a mid boss it feels like an epic battle. In between you get to romance, or fly around helping someone.... etc. The world feels bigger, with more in it.
IN ME3, I felt like all enemies were kinda the same. Except the banshees at the asari ardak-jasi (or something like that) temple where it felt more dreadful.
Also, no final boss in ME3 (This was something I just dont understand).
Perhaps the problem is that in both ME1 and 2 there was something that really impressed me, and I overlooked the things that would bother me.
Ofc thats my view of it, not a fact in itself.
In a way I felt like ME3 was more like Call of Duty, less like Mass effect (Many cool scenes to make it feel like a movie instead of a nice challange to make you feel like you are the one doing the great stuff).